Rather than setting emission standards for each source of pollutant from a plant it is possible to set an upper limit for all emissions irrespective of origin. This is known as the ‘bubble’ concept: a notional bubble is drawn around a plant or area and an upper limit is put on the total amount of a pollutant allowed to pass into the bubble. Thus if a manufacturer succeeds in reducing his diffuse discharges he may emit more through a chimney stack and vice versa. The concept can be extended to an area covering several manufacturers in which case market forces may lead them to sell and buy among themselves the right to emit pollutants so long as the total does not exceed that prescribed. Thus a new manufacturer may have to pay existing polluters to reduce their emissions in order to create the ‘space’ for himself. This is also known as the ‘emission offset policy’. The concept can be extended to a whole country or even to the whole Community or indeed globally. An upper limit has been set for the emission of sulphur dioxide from large combustion plants in each Member State (see Section 6.10) and the Community in the past set an upper limit on the total production and thus effectively the emission, of CFCs (see Section 6.12). Production of CFCs is now banned. A total emission limit for such a versatile material as lead has not so far been suggested. However, bubbles do form part of the basis for national commitments under the Kyoto Protocol to the UN Convention on Climate Change (see Section 13.2) and are central to the proposed national emission ceilings Directive (see ‘Legislation awaiting adoption’).